The present invention relates to an ultrasonic diagnostic apparatus and, more particularly, to an ultrasonic diagnostic apparatus for transmitting an ultrasonic wave to the object to make a diagnosis for the object on the basis of the reflected wave.
There is widely known a ultrasonic diagnostic apparatus for transmitting an ultrasonic wave from an ultrasonic transducer incorporated in a catheter inserted in a blood vessel or a vessel, receiving the wave reflected by the tissue inside the object by the ultrasonic transducer, performing detection, amplification, and the like for the reflected wave, and displaying an image based on the reflected wave on a display such as a CRT.
In general, an ultrasonic wave radiated inside the object is absorbed and attenuated by the tissue of the object in passing through a diagnosis region. The absorption coefficient of a living body is about 1 to 3 dB/(cm.multidot.MHz). The influence of absorption and attenuation depends on not only the propagation distance of the ultrasonic wave, but also the frequency of the ultrasonic wave. A higher-frequency ultrasonic wave attenuates more than a lower frequency ultrasonic wave.
The resolution along the propagation direction of the ultrasonic wave in the ultrasonic diagnostic apparatus becomes higher as the frequency of the ultrasonic wave becomes higher. On the other hand, as the distance from the ultrasonic transducer increases, the sensitivity decreases due to absorption and attenuation of the ultrasonic wave, resulting in an image having a low signal to noise ratio (hereinafter. referred to as "S/N ratio").
A high-frequency ultrasonic wave is therefore used for a near-distance portion from the ultrasonic transducer to obtain a high-resolution image, whereas a low-frequency ultrasonic wave is used for a far-distance portion to obtain an image in which the S/N ratio hardly decreases though the resolution is low. According to this method, a wave reflected by the tissue inside the object is received by an ultrasonic transducer, and the characteristics of a filter for filtering the reflected wave are changed in accordance with the distance from the ultrasonic transducer (observation depth).
As this filter, a bandpass filter (BPF) obtained by combining a low-pass filter (LPF) and a high-pass filter (HPF) is generally used. The low-pass filter decreases the cutoff frequency as the observation depth increases, thereby realizing a high resolution in a region where the observation depth is small, and preventing a decrease in S/N ratio in a region where the observation depth is large. The high-pass filter decreases the cutoff frequency as the observation depth increases. By virtue of these characteristics on a LPF and a HPF, the bandpass filter suppresses the thermal noise to a predetermined value or less, while it makes the bandwidth constant.
In an electrically scanned ultrasonic diagnostic apparatus, the use frequency is about 3.5 to 10 MHz. To realize a filter having characteristics variable in accordance with the observation depth at these frequencies, the low-pass filter and the high-pass filter are constituted by one RC or LC circuit, and a variable capacitance diode is employed as a capacitor (C).
As the use frequency exceeds 20 MHz like a celiac ultrasonic diagnostic apparatus for representing a blood vessel and a vessel as an image, the band broadens, thus resulting in increasing received thermal noise. This is because the RMS (Root Mean Square) value of the thermal noise is proportional to the square root of the bandwidth.
In the above-described filter having a variable cutoff frequency, the frequency cutoff characteristics must be made steep to eliminate the influence of redundant thermal noise. However, in the above-described filter constituted by one RC or LC circuit, the frequency cutoff characteristics are poor, and a practical diagnosis image cannot be obtained.